Evoked potential measurements are now commonly made in clinical practice and in research to evaluate nervous system functions. To measure the evoked potentials stimulated by electrical pulses, surface measurement electrodes are customarily positioned on the scalp or skin over peripheral nerves. The electrical potentials received by these electrodes are detected and analyzed by sensitive recording equipment. To stimulate a response in the nervous system, stimulation electrodes are applied to the skin of the subject at a position remote from the measurement electrodes, typically on an arm or leg, and a pulse of either constant voltage or constant current magnitude is then applied to the individual between the two stimulation electrodes.
A ground strap or electrode is conventionally placed on the individual being tested at a position between the stimulation electrodes and the measurement electrodes. The grounded electrode maintains the mean voltage of the limb to which it is mounted near ground and prevents current passing through vital organs if the stimulation equipment should short circuit. The electrical circuit which delivers the stimulation pulse is electrically isolated from ground so that no net current should pass through the individual to ground from the stimulating electrodes. However, it is observed with conventional stimulation instruments that the large common mode transient voltage appearing at the electrodes at the time of a pulse causes displacement current to flow through the inherent parasitic capacitance of the patient to the patient referenced ground or common. This "escape" displacement current creates a voltage transient at the measurement electrodes that can saturate the physiological amplifiers for several milliseconds following the stimulus pulse, thereby obscuring or interfering with the recording of pertinent evoked potential data. See K. C. McGill, et al., "On the nature and elimination of stimulus artifact in nerve signals evoked and recorded using surface electrodes," IEEE Trans. Biomed. Engr., Vol. BME-29, No. 2, pp. 129-137, February 1982.
As explained in the foregoing article, the common mode voltage at the recording site is the sum of (1) the voltage dropped between the grounding and recording sites by the stimulus current, and (2) the voltage dropped across the ground electrode by the escape current. At the rising and falling edges of the stimulus pulse, the stray stimulator capacitances must be charged and discharged. Unless the capacitances and the stimulating electrode impedances are perfectly balanced, this results in escape current flow through the ground electrode.